The dc, small-signal, and microwave power output characteristics of AlGaN/GaN HEMTs are presented. A maximum drain current greater than 1 A/mm and a gate-drain breakdown voltage over 80V have been attained. For a 0.4 m gate length, an of 30 GHz and an max of 70 GHz have been demonstrated. Trapping effects, attributed to surface and buffer layers, and their relationship to microwave power performance are discussed. It is demonstrated that gate lag is related to surface trapping and drain current collapse is associated with the properties of the GaN buffer layer. Through a reduction of these trapping effects, a CW power density of 3.3 W/mm and a pulsed power density of 6.7 W/mm have been achieved at 3.8 GHz.

The two deep traps responsible for current collapse in AlGaN/GaN high electron mobility transistors grown by metalorganic vapor-phase epitaxy have been studied by photoionization spectroscopy. Varying the growth pressure of the high resistivity GaN buffer layer results in a change in the deep trap incorporation that is reflected in the observed current collapse. Variations in the measured trap concentrations with growth pressure and carbon incorporation indicate that the deepest trap is a carbon-related defect, while the mid-gap trap may be associated with grain boundaries or dislocations.

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Photoionization spectroscopy (PS) measurements, previously carried out for the GaN MESFET, have been extended to the more complicated AlGaN/GaN HEMT structures. In all cases, the spectra revealed that the same two traps causing current collapse in the high resistivity (HR) GaN buffer layer of the MESFET were also responsible for current collapse in the HEMT structures. The HR buffer layers supporting the HEMT structures were prepared by MOVPE at varying growth pressures, in order to vary the incorporation of deep trapping centers. Lower growth pressures were observed to correlate with more severe current collapse and with an enhanced incorporation of carbon. Detailed analysis of the PS data suggests that one of the two responsible traps is related to carbon, while the other may be associated with structural defects in the material.